The present invention relates to a fuel reforming system in a mobile fuel cell system represented by a fuel cell automobile.
A prior art of a fuel reforming system in a mobile fuel cell system represented by a fuel cell automobile is explained by referring to FIG. 1. A mixed solution 101 of water and methanol used as fuel contained in a fuel tank 100 is sent into an evaporator 102, and heated and evaporated to form a mixed vapor 103 of water (steam) and methanol, and sent into a mixer 104. Air 106 is also sent into the mixer 104 from a compressor 105. The mixed steam 103 and air 106 are mixed in the mixer 104, and sent into an autothermal reforming reactor (ATR reactor) 107.
The ATR reactor 107 reforms the methanol used as fuel by catalytic reaction shown in chemical reaction formulas Chem. 1 and Chem. 2 by using water and oxygen in air, and produces an oxygen-rich reformed gas.CH3OH+H2O→CO2+3H2−Q1  (Chem. 1)CH3OH+½O2→CO2+2H2+Q2  (Chem. 2)
Chemical reaction formula Chem. 1 shows steam reaction of methanol (endothermic reaction), and chemical reaction formula Chem. 2 denotes partial oxidation reaction of methanol (exothermic reaction). The reaction in chemical reaction formula Chem. 1 progresses in steps mainly as shown in chemical reaction formulas Chem. 3 and Chem. 4.CH3OH→CO+2H2−Q3  (Chem. 3)CO+H2O→CO2+H2+Q4  (Chem. 4)
Chemical reaction formula Chem. 3 shows decomposition reaction of methanol (endothermic reaction), and chemical reaction formula Chem. 4 denotes shift reaction of carbon monoxide (exothermic reaction). The ATR reactor 107 is operated in the autothermal condition for balancing the exothermic and endothermic reactions in these formulas. Therefore, once the size or structure of the reactor, or the performance of the catalyst is determined, the flow rate of steam in relation to the flow rate of the fuel methanol, and the flow rate ratio of air flow rate are almost determined.
The rate of the methanol flow rate consumed for partial oxidation reaction (POX) corresponding to the total methanol flow rate being supplied is defined to be POX rate. Since almost all amount of supplied oxygen is consumed in the partial oxidation reaction (POX) in formula 2, the air flow rate necessary for the total methanol flow rate is determined from this POX rate.
At the starting time when a catalyst temperature is low, subsidiary reactions given in the following chemical reaction formulas Chem. 5 and Chem. 6 take place at the same time.CH3OH→HCHO+H2  (Chem. 5)HCHO→CO+H2  (Chem. 6)
Operating temperature of the ATR reactor 107 is 300 to 600° C., and from the thermodynamic chemical equilibrium, reformed gas containing several percent of carbon monoxide is obtained. Carbon monoxide poisons the fuel electrode catalyst composed of platinum and others of solid polymer type fuel cell (FC) 200, and lowers its activity extremely, and therefore it is necessary to decrease the concentration of carbon monoxide to tens of ppm to 100 ppm by a carbon monoxide remover composed of a shift reactor 108 and selective oxidation reactor (PROX reactor) 109, and then supply into the fuel cell 200.
The reformed gas containing several percent of carbon monoxide is sent into the shift reactor 108, and the carbon monoxide is decreased by the catalyst quick in the shift reaction of chemical reaction formula 4. Operating temperature of the shift reactor 108 is 200 to 300° C., and from the thermodynamic chemical equilibrium, reformed gas containing several percent of carbon monoxide is obtained. The shift reactor 108 may be integrated with the ATR reactor 107. The reformed gas decreased in the carbon monoxide by the shift reaction is sent into the PROX reactor 109, and the carbon monoxide is further decreased to tens of ppm to 100 ppm at most by the catalytic oxidation reaction (exothermic reaction) in the following chemical reaction formula 7. The required oxygen is supplied from the compressor 105 as air.
                              CO          +                                    1              2                        ⁢                          O              2                                      →                              CO            2                    +                      Q            7                                              (                  Chem          .                                          ⁢          7                )            
In the PROX reactor 109, oxidation reaction is performed in steam atmosphere, and the following hydrogen combustion reaction (exothermic reaction) takes place at the same time. Accordingly, the selectivity of chemical reaction formula Chem. 7 on chemical reaction formula Chem. 8 has a serious effect on the efficiency of fuel reforming system.
                                          H            2                    +                                    1              2                        ⁢                          O              2                                      →                              2            ⁢                          H              2                        ⁢            O                    +                      Q            8                                              (                  Chem          .                                          ⁢          8                )            
The PROX reactor 109 removes heat generation by chemical reaction formulas Chem. 7 and Chem. 8, and maintains operating temperature at hundred and tens of degrees centigrade, and therefore it is cooled by air, LLC (coolant), or oil, although not shown in the diagram.
Thus, the reformed gas reduced in carbon dioxide to an extremely low concentration and air from the compressor 105 are sent into the fuel electrode and air electrode of the fuel cell (FC) 200, and power is generated.
In the fuel cell 200, it is impossible to use all of the hydrogen contained in the reformed gas, and the reformed gas used in power generation containing a partial residual hydrogen, and the air used in power generation containing a partial residual oxygen are sent into a catalytic combustor 110, and burn. The obtained high temperature exhaust gas is sent into the evaporator 102, and recycled as evaporation energy of methanol and water.
Reference numeral 500 is a flow rate control valve for controlling the flow rate of the air supplied into the PROX reactor 109, 501 is a flow rate control valve for controlling the flow rate of the air supplied into the ATR reactor 107, and 502 is a flow rate control valve for controlling the flow rate of the air supplied into the air electrode of the fuel cell 200.
Reference numeral 510 is a pressure control valve for adjusting the operating pressure of the fuel electrode of the fuel cell 200, and 511 is a pressure control valve for adjusting the operating pressure of the air electrode of the fuel cell 200. Reference numerals 520 and 521 are pressure sensors for detecting the operating pressure at the fuel electrode side and air electrode side of the fuel cell 200, and the pressure is adjusted so that the pressures of these may be equal.
Reference numeral 400 is a controller for mobile energy management, and it sends an operation load signal 402 of the fuel reforming system to a fuel cell controller 401. The fuel cell controller 401, on the basis of the operation load signal 402, drives a pump 111 so as to achieve the flow rate of fuel vapor and air necessary for the ATR reactor 107, and controls the flow rate of the liquid fuel to be supplied into the evaporator 102 and controls the flow rate control valve 501. Reference numerals 601 and 602 are flow rate sensors.
As the evaporator 102, if a huge evaporator capable of controlling temperature of the generated vapor thereof is used, or if an evaporator of an extremely large thermal capacity or heating area is operated at a constant temperature, vapor temperature supplied from the evaporator 102 into the ATR reactor 107 can be kept substantially constant.
Such evaporator may be used in a fuel reforming system for a stationary fuel cell, but the fuel reforming system for mobile fuel cell is limited in space and such evaporator cannot be installed, and the evaporator 102 of compact and simple structure have to be used.